The present invention relates to a method for bonding insulating members such as resin parts on an industrial scale and in an economical process. More particularly, the present invention relates to a bonding method of a resin part, which enables use of a heating device which is safe in operation, stable in quality and free of hindrance by radio waves as it is made possible according to this method to use a high-frequency power source for heating with a relatively low easily available frequency, and to the heating device therefor.
High-frequency induction heating method is known as a heat-bonding method for resin parts. This method is widely used in the industries as it is capable of uniform heating of conductive materials. For instance, this method is used for laminating insulating members of rubber or plastic on a conductive product such as steel pipe and steel plate, etc. It is further applied to bonding of the insulating members both of which are made of resin. In this case, since the resin is an insulating material, a conductive composite material containing a conductive substance such as iron powder as susceptor is incorporated as a heating-medium between resin parts to be bonded of the resin articles, and an alternating electromagnetic field with a high frequency of 1 to 13 MHz is applied to the said heating-medium, thereby heating and fuse-bonding the insulating member.
With advancement of application of various resins as industrial materials in recent years, the request has become acute for the improvement of the method of heat-bonding of the insulating members both of which are made of resin or the like. More specifically, it has become essential to reduce the thickness and size of the heating-medium itself in conformity to the reduction of thickness and size, and complication of geometry of the resin articles to be bonded, and therefore, an offer of a heating-medium having an excellent bonding effect is strongly demanded.
Further, the decrease of content of the susceptor material which remains at the parts to be bonded (bonded parts) and tints blackish or brownish color is keenly required for realizing effective application of the bonding techniques for resins which have seen diversification in aesthetic quality such as light-coloring transparency, etc.
A conductive metal powder is contained in the heating-medium used in the said bonding method by high-frequency induction heating. Iron powder is commonly used as the said conductive metal powder. Generally, the iron powder having small particle size tends to be oxidized, thereby gathering rust and lowering the conductivity thereof, so that there are used the relatively large particles which resist rust.
This fact is mentioned in, for instance, Japanese Patent Publication No. 52-65 (1977), which states: "In the conventional induction heat-sealing methods, iron powder with relatively large particles, viz. the particles of 0.05 to 20 mils (0.001 to 0.5 mm) in diameter, has been generally used as susceptor in the composition of heating-medium."
Rusting of the particles causes unfavorable phenomena such as lowering of heat-generating effect and reduction of bonding strength. Therefore, a rustproof material such as magnetic iron oxide particles or ferrite particles has been used as susceptor by some manufacturers. However, when using such a susceptor in the said high-frequency induction heating method, it becomes difficult to generate an eddy current because the magnetic iron oxide particles or ferrite particles are a high-resistivity material, so that such magnetic iron oxide particles or ferrite particles are inferior in heating efficiency to metal powder such as iron powder for heating-medium.
The conventional high-frequency induction heating is intended to convert the imposed electric energy into heat energy by inducing an eddy current in the heating-medium, so that it was necessary to use a high-frequency (several MHz) power source equipment. Also, in case of using a heating-medium comprising a magnetic complex of iron oxide particles or ferrite particles and a resin, it was necessary to use a high-frequency power source with a higher frequency than that required when using metal powder such as iron powder in order to generate eddy current.
Thus, the conventional methods have the various problems such as insufficient operational safety and quality due to high frequency required for the high-frequency power source and the necessity of providing precautions for preventing radiation hazard.
As noted from the above statement, an offer of a resin-bonding method which is capable of effecting heat-bonding of insulating materials such as resins in a short time and enables use of a bonding apparatus which is safe in operation, stable in quality and free from radiation hazard, and a heating device used for such resin-bonding, are strongly demanded.
As a result of the present inventors' studies for fulfilling the said requirements, it has been found that it is possible to perform desired bonding in a short time by using a high-frequency power source with a relatively low frequency and a method comprising disposing a magnetic complex composed of a resin and magnetic particles as a heating-medium between a resin part and a insulating part, and applying an alternating magnetic field to the said heating-medium by using a magnetic field generator of a relative low frequency as a heating unit for resin-bonding composed essentially of an electromagnetic core and an exciting coil to make the said heating-medium generate heat. The present invention was achieved on the basis of the above finding.